Fiber reinforced plastic (FRP) composites have found increasing usage as a replacement for metal and other structural materials especially with automotive and aerospace industries due to their high strength and low weight. Epoxy resins have been the matrix resins generally used in high performance FRP composites. Carbon fiber, epoxy matrix composites are routinely used for secondary structural applications in contemporary aircraft. However, epoxy resins typically exhibit poor strength at elevated temperatures after aging in humid environments, i.e. they exhibit depressed glass transition temperatures (Tg), especially after thermal cycling under very humid wet conditions.
State-of-the-art epoxies for matrix applications, such as those based on tetraglycidyl methylenedianiline (TGMDA) cured with aromatic diamines, such as 4,4'-diaminodiphenylsulfone (4,4'DDS), present structural processing problems such as high flow, and have poor properties such as moisture sensitivity and low hot wet strength, poor fracture toughness and low impact strength. New materials are needed that offer simpler cure cycles, lower flow, and that exhibit better toughness and storage properties at temperatures above 0.degree. F.
Bis-maleimide resins exhibit good humidity resistance and are capable of use as matrix resins in humid, high temperature environments. However, the available bis-maleimide resin systems have generally exhibited poor handleability, brittleness and long cure times even at high curing temperatures up to 475.degree. F. Carbon and high modulus graphite composites cured with these resins have exhibited severe microcracking after environmental aging due to thermal, mechanical or physical gradients between the resin and fibers.
A bis-imide matrix resin system comprising 50 to 95 percent by weight of ethylenically unsaturated bis-imides, preferably a low melting mixture of a major portion of maleimides of aromatic amines with a minor portion of maleimide of an aliphatic amine, and 5 percent to 35 percent by weight of a di-unsaturated low-temperature cross-linking agent such as divinyl benzene which gels the bis-imide at low temperatures is described in U.S. Pat. No. 4,351,932. This composition is said to reduce stress between the matrix resin and the surface of the reinforcing fiber, thus reducing the tendency to form microcracks. Microcracking is said to be further reduced and transverse strength increased by the addition of 0 to 15% of compatible elastomers to the resin and cross-linking agent. Heat resistance and cross-link density are said to be improved by the presence of 0 to 10% of a trifunctional curing agent. The divinyl benzene contributes to room temperature tackiness but it is undesirable to use divinyl benzene commercially because it compromises handling due to volatility and odor.
Low temperature curable compositions of bis-maleimide and epoxy are described in U.S. Pat. Nos. 4,273,916 and 4,293,521.
U.S. Pat. No. 4,273,916 describes a prepolymer comprised of the reaction product of at least one aliphatic bis-maleimide, at least one aromatic amine and at least one aromatic bismaleimide that is combined with at least one cycloaliphatic epoxy resin having a melting point less than about 120.degree. F. and at least two functional epoxy groups to provide a low temperature curable composition. U.S. Pat. No. 4,283,521 describes a prepolymer comprised of the reaction product of at least one aliphatic bismaleimide, at least one aromatic amine and at least one aromatic bismaleimide in combination with an aromatic and/or a cycloaliphatic epoxy resin providing at least two functional epoxy groups to provide a low temperature curable composition.
U.S. Pat. No. 4,211,861 describes thermosetting imide resins that are obtained by reacting an N,N'-bis-imide of an unsaturated dicarboxylic acid with the hydrazide of an amino acid, preferably in the molar proportion between about 1.1 and about 10.0. The resulting prepolymerization products can also be prepared in organic solvents or diluents. By heating at atmospheric pressure or under pressure to a temperature between about 100.degree. C. and about 350.degree. C. and preferably between about 160.degree. C. and 260.degree. C., if desired, in the presence of curing catalysts or inhibitors, the prepolymerization products are cured and hardened to substantially insoluble, infusible, highly cross-linked imide resins.
U.S. Pat. No. 4,269,966 describes polyimide prepolymers that are produced by reacting an unsaturated dicarboxylic acid imido acylchloride with a difunctional amine to produce the corresponding acid amide. Condensation of said reactants is preferably effected in solution in a low boiling solvent. The resulting prepolymer is hardened and completely polymerized by heating, preferably between about 80.degree. C. and about 400.degree. C., to yield a cross-linked, substantially infusible and insoluble polyimide resin.
The increasing use of composite materials in primary structures as well as in secondary structures is changing the prepreg market demands. Because higher volume production requires more efficiency, greater yields and automation, processors are in search of prepregs which can offer: oven curability or compatibility with existing autoclaves (as opposed to requiring high temperature and/or high pressure autoclave); low pressure cure (14 psi); room temperature storage; low but consistent tack (e.g. for automatic laydown machines); low bleed; and suitable adhesive properties of the prepreg.
Designers are looking for: non flammability; low smoke and toxicity; improved impact; repairability; improved hot wet compression and moisture resistance at 350.degree. F.; and thermal and mechanical stability at 450.degree. F. Polymers or formulations available to date are deficient in one or more of the desired properties.
Even though the properties of epoxy resins are less than desired, there has been no known suitable high temperature replacement that offers better cost effective performance than epoxy resins. Thus, new resins and resin formulations are needed, particularly for use at higher temperatures and in primary structures where long-term durability is a major concern.